The effect of subcutaneous naloxone administration on the consumption of a weak ethanol solution in rats on the three consecutive days (testing days) was investigated using a behavioral paradigm which includes a first forced ethanol exposure (conditioning day) followed by a two-bottle ethanol/water choice procedure. Besides reducing fluid intake, naloxone treatment prior to forced ethanol exposure interferes with the acquisition of ethanol preference. Post-conditioning naloxone administration fails to affect ethanol preference. Administration of naloxone prior to the first testing session induces a reduction on total fluid intake, at the day of treatment; a decrease on ethanol preference throughout the three consecutive testing days is also observed with the higher dose of the antagonist (5 mg/kg). An involvement of endogenous opioids in ethanol consumption is suggested through the modulation of alcohol reinforcement or the affective quality of the gustatory cue.